There have been proposed various techniques for optimizing viewing angle of a display, such as a rear projection display or a liquid crystal display, and improving contrast in images displayed by the display so that the viewers are able to recognize images clearly.
More concretely, a previously proposed first optical element is provided with lenticular lenses, namely, light diffusing elements, on its back surface (a surface facing an image light source) on which image light falls, and with light absorbing elements on light intercepting parts, which do not transmit image light, in its front surface on the viewing side to absorb external light incident on the front surface.
A second optical element proposed in JP 50-121753 U and JP 60159733 A is provided on its front surface on the viewing side, as light diffusing elements, with many ribs each having inclined reflecting facets and an exit facet formed between the inclined reflecting facets, and a light absorbing material filling up V-grooves between the adjacent inclined facets to absorb external light incident on the front surface from the viewing side.
Although the first optical element is effective to some extent in improving contrast, the first optical element cannot be used as a directional optical element because parallel light rays perpendicularly incident on the first optical element are diffused by the lenticular lens elements. In the first optical element, the lenticular lens elements on the back surface and the light absorbing parts on the front surface need to correspond to each other in one-to-one correspondence. Therefore, it is very difficult to manufacture the first optical element when the lenticular lenses are arranged at short pitches.
The second optical element divides incident light rays into outgoing light rays that are emitted through the front surface respectively in three directions, namely, light rays not reflected by the inclined facets of the ribs and emitted through the exit facets in a direction normal to the exit facets, light rays totally reflected by the opposite inclined facets of each rib and emitted through the exit facet in two directions at specific angles to the exit facet. Therefore, the ribs through which light rays travel must contain a large amount of light diffusing particles to display uniform images. The large amount of light diffusing particles spoils the clearness of images and hence the second optical element cannot be used as a directional optical element.